K Mean Black

• Name: Printing Technology
• Branch: Printing Technology Diploma 6th Sem
• Published: May 25, 2025

Quality Management Part - 1

Unit – 4

4.2 SPECTROPHOTOMETERS                           

• A spectrophotometer is a device for measuring light intensity by measuring the wavelength of light.
• The most common application of spectrophotometers in the printing industry is the measurement of light absorption.

Working principle of Spectrophotometers:

1. Place the spectro on the sample and click the button to measure.

2. The spectro projects a light onto the sample’s surface.

3. The sample absorbs some of the light and reflects the rest into a set of filters.

4. The spectrophotometer measures that reflected light at many different points to capture the fingerprint, or signature, of the sample and presents it as a reflectance curve.

5. The reflectance curves of samples can be compared to the reflectance curves of standards to determine even the most minute color differences.

6. This is how manufacturers determine whether the color they’re producing is on target.

 

 

 

 

 

Basic components:

1) Light source

• Two kinds of lamps, a Deuterium for measurement in the ultraviolet range and a tungsten lamp for measurement in the visible and near-infrared ranges, are used as the light sources of a spectrophotometer.
• The standard illuminants A, C, D50 or D65 and the standard observers 2° and 10° can be used.

 

Spectroscope

• A spectroscope plays a role in selecting a monochromatic light from a light source (white light).
• Spectroscopes include Filter type, Prism type, and Grating (diffraction grating) type.

 

Optics

• A container that contains a sample is usually called "cell"; two types are available, glass and quartz cells.
• A glass cell used for measurement in the visible range of 340 nm or more.
• A quartz cell used for the measurement in the ultraviolet range due to its high price.

 

Detector

• A detector used to convert the light transmitted from a sample into an electric signal.
• Optical semiconductor, Types of photomultiplier, etc. are available.

 

 

REFLECTANCE CURVES

• A spectrophotometer produces a reflectance curve that can indicate the color being measured. White light contains all the colors of the rainbow in the visible spectrum.
• When white light falls upon an opaque object, the object interacts with that light.
• If the object appears red it is because the object absorbed most of the white light selectively, reflecting only the red portion, which is observed by the eye.
• Likewise, if an opaque object is green, it absorbs all of the white light except the green portion.
• White objects produce a reflectance curve which is essentially flat at nearly 100 percent (reflecting all wavelengths), and black objects produce a curve which is essentially flat at nearly 0 percent (absorbing all wavelengths).

 

1. 3. Analysis of Print Attributes

 

1) Solid Ink Density (SID)

• Density is the ability of a material to absorb light. Generally, the darker a process color is to the eye, the higher the density.
• Solid ink density is the measurement of a solid printed patch on the paper, including the paper density.
• Density is caused by the light-stopping ability of the pigments in the printing ink that are deposited on the paper by the printing process.
• Densitometers are widely used in the graphics industry to help control color in each step of the printing process.
• Density measurements of solid ink patches are used to monitor the ink film thickness applied during a press run.
• In comparing two printed sheets, density readings should be within .05 units, when measured on a densitometer, for meaningful print quality assessment.

• Dot gain, print contrast and apparent trap are directly affected by this solid ink density. Generally, these values will vary as the solid ink density changes.
• The readings of a solid area, are referred to as solid density. It is measured on a print control strip, which is printed on the sheet at right angles to the print direction.
• The solid density value allows a regular ink film thickness to be checked and maintained (within a certain tolerance) throughout the whole sheet width and printrun.
• Therefore, for most press operators, the minimum requirement for a color contains solid patches of the inks that will be printing since solid ink density is the only thing on press that an operator can adjust while the press is running.
• Those solid patches are then repeated over the width of the press sheet so that each ink zone is represented by at least one complete set of patches - containing one patch for each color being printed.